Manufacture of diolefins



Patented Mar. 7, 1944 UNITED STATES PATENT OFFICE LMANUFACTURE 0F DIOLEFIN S Robert F. Rnthrufl', Chicago, 111.

N 0 Drawing.

Application November 17, 1942,

Serial No. 465,904 6 Claims. ('01. 260-680) This invention relates to the preparation of diolefins from monoolefins. More particularly, this invention relates to the production of conjugated diolefins from monoolefins containing four to five carbon atoms to the molecule.

Thi application is in part a continuation of my copending application, Serial Number 299,695, filed October 16, 1939, which has issued in the pyrolysis of propane, in addition to propene and hydrogen, ethene and methane are observed in the products, the latter pair usually being present in the greater amount. Similarly, normal butane on pyrolysis,1 addition to giving butene and hydrogen, also yields propene and methane as well as ethene and ethane. In order to repress these pyrolytic decomposition reactions involving the rupture, of a carbon to carbon bond and at the same time promote the dehy drogenation reaction, resort is usually had to catalysts. Among suitable catalysts for the purpose may be mentioned. alumina and alumina containing minor proportions of an oxide of a sixth group metal such as chromium oxide and molybdenum oxide. When. such catalysts, or

- similar ones, are employed the dehydrogenation reaction proceeds practically to the exclusion ofh .40 carbon bond rupture. ,Selectivities of from 80% f decomposition reactions involving carbon to tend to assume this configuration practically to the exclusion of all other possible products.

While the above discussion has been directed almost exclusively to the behavior of parafiins when subjected to pyrolysis or catalytic dehydrogenation, other hydrocarbons, such as olefins and cycloparaflins behave in an analogous manner on pyrolysis or catalytic dehydrogenation. For example, by the pyrolysis of the normal butenes, more or less butadiene 1,3 is formed but by far the greater part of the charge reacting gives other products. When, however, catalysts such as those previously mentioned, or others exhibiting similar properties, are em ployed, the yield of butadiene' 1,3 is much ennanced. When olefins containing six or more carbon atoms, especially those containing six or more'carbon atoms in a straight chain, are dehydrogenated the ring forming reaction is again the principal one observerd. Also, if a cycloparafiin, such as cyclohexane, is subjected to catalytic dehydrogenation benzene is formed alto 0% or even more are commonly. observed.

(Selectivity may befdefined as the percentage of the charge reacting that reacts to form an olefin having the same number of carbon atoms as the charge.) j

When an attempt is made to catalytically dehydrogenate a hydrocarbon "containing six or more carbon atoms, especially a hydroca rbon containing six or more carbon atoms in a straight chain, an entirely new reaction is observed.

Only a small amountbf olefins is, produced; aromatic hydrocarbons being theprincipal products. Because of the'peculiar energy relationships of the benzene ring, the reaction products most quantitatively.

As the principalobject of this invention i the production of conjugated diolefins'from monoolefins it is obvious that the number ofsuitable charging stocks is considerably circumscribed. Since a conjugated diolefin must contain four or more carbon atoms, suitable monoolefins for the.

production of these dienes must also contain four or more carbonatoms. Similarly, since monoolefins containing six or more carbon atoms, especiallythose containing six or more carbon atoms in a straight chain, react almost exclu-- sively-to produce aromatics, itis evident that suitable straight chain monoolefins for the purposes of this invention preferably contain five carbon atoms or less. Hence, the preferred charging stocks for the purposes of this invention, are essentially limited to the butenes,

especially the normal butenes, and the pentenes,

especially the normal pentenes and the methyl pentenes, Branched chain monoolefins of six or more carbon atoms, but having no more than five carbon atoms in the longest straight chain can be used in the practice of this invention but even these materials, when subjected to catalytic dehydrogenation, isomerize to a considerable extent with the eventual production of aromatics so the employment of such monoolefines as charging stocks for the instant invention is oftennot practical. Cycloparafiins, such as cyclobutane, cyclopentane and methyl cyclobutane are identical in compositionand functionally the four and five carbon atom monooleflns and can be substituted therefor in the practice of this invention. 7

In the dehydrogenation of a monoolefln to produce 'a conjugated dioleiin the general reaction may be expressed as follows:

to 30% or more based on the conjugated .diolefln formed. This high carbon production obviously represents a direct loss of the charge but it has even more serious consequences. The large amount of carbon formed is deposited on the surfaces of the catalyst thereby rapidly destroying its activity and necessitating frequent regenerations. Actually, when operating at subatrnospheric pressures and under the time-temperature conditions necessary for appreciable conversions of monoolefins to dioleflns, the activity of the catalyst declines so rapidly that the possible onstream cycle is so short as to make the process uneconomic. The possible on-stream cycle can 15 be appreciably increased by reducing the severity crease in volume the reaction will be favored by operating at low pressures. The extent of the reaction will increase if one of the products of reaction is continuously removed from the reaction zone. The essence of the instant invention is the a discovery that more satisfactory results are obof the operating conditions, for example by decreasing the contact time or the temperature or both, but when this is done the conversion of the monoolefin to the conjugated diolefin is so 2. low that again the process is uneconomic.

It has now been found that when the monoolefinic charge is diluted with hydrogen and the mixture is passed, preferably at superatmospheric ressure. over a suitable catalyst at elevated tem- P 25 peratures, high conversions to conjugated diolepreferably at subatmospheric pressure, it has been I found that more satisfactory results are obtained 30 by operating at atmospheric pressure or preferably at superatmospheric pressure. Instead o removing one of the products or the reaction. it has been found that more satisfactory results are obtained by deliberately adding a large amount of one of the products of the reaction (hydrogen) to the charge. It should be understood that no claim is made that the low of mobile equilibrium does not hold in the present instance. It has been found that the major reaction is accompanied by certain highly unfavorable by-reactions and that these by-reactions are greatly accelerated by operating in accord with the teach ings of the law of mobile equilibrium with respect to the major reaction. By operating in a manner diametricallyopposed to the teachings of the law of mobile equilibrium the unfavorable by-reactions are largely or completely suppressed and the principal reaction, while somewhat suppressed. proceedsinahighlysatisfactorymanner. I One object of the instant invention is to provide an improved process for the production of conjugated dioleflns by the catalytic dehydrogenation of monoolefln's. A further object of this invention is to provide a process for the production of conjugated diolefins by the catalytic define are obtained and at the same time the production of carbon is greatly reduced. By operating in the presence of added hydrogen, and preferably under pressure, very satisfactory conversions of monooleilns to conjugated dioleiins are obtained and at the same time the deposition of carbon on the catalytic surface is so slow that on-stream cycles of economic length are achieved. It is evident that the slow deposition of carbon is equivalent to a low yield of carbon based on the monoolefins charged, a result that is also favorable for the success of the process.

It is believed that when monoolefins are dehydrogenated in the presence of added hydrogen,

4. especially when the reaction is conducted at superatmospheric pressure, the large excess of hydrogen tends to suppress carbon formation by reversing the reaction:

according to the teachings of the law of mobile equilibrium. Also it is believed that the large excess of hydrogen tends to clean up any carbon that may be deposited in the nascent state on the hydrogenation of monooleilns in the presence of added hydrogen. An additional object of this invention is to provide a process for the production of conjugated dioleflns by the catalytic de-' hydrogenation of monooleflns in the praence of added hydrogen and at elevated pressures. Other objects of this invention will become evident from a perusal of the instant specification.

In the catalytic dehydrogenation of a monoolenntoformaconiugateddioleflnunderthe conditions prescribed from a consideration of the law of mobile equilibrium a large amount of carbon forms as a byproduct. For example, when normal butenes are catalytically dehy at elevated temperatures and subaflnospheric pressure, under the time-temperature conditions necessary to givehigh yields of butadiene 1,3 the under pressure, is theory only and in no way limits catalytic surfaces through the mechanism expressed by the type reaction:

Any carbon that is deposited when operating under the improved conditions is laid down in the. nasoenirstate on catalytic surfaces having high hydrogenating-dehydrogenating properties. These surfaces probably hold an adsorbed layer of hydrogen in a highly active state so that the removal of nascent carbon atoms with this adsorbedhydrogenistobeexpected. Itistobe understood that this explanation of the favorable action of added hydrogen, especially hydrogen the scope of this invention.

For more specific details and examples of the instant invention reference may be made to the following discussion.

'While the catalytic dehydrogenation of monoolenns in the presence of added hydrogen and at atmospheric or preferably superatmospheric pressure to produce conjugated diolenns has been generically described in my copending application,

carbon simultaneously produced amounts to 5% 15 previously mentioned, the present application is particularly directed to operations conducted under'a narrow range of conditions within thepreviously outlined broad rangeof conditions. When operating within said narrow range of conditions, to be particularly set forth immediately hereinafter, it has been found that highly beneficial and unexpected results are obtained. Briefly, said narrow range of conditions embraces:

1. Operations in the presence of at least one mole and not over ten moles of hydrogen-per mole of monoolefin charge, preferably in the presence of from three to five moles of hydrogen per mole of monoolefin charge, and

2. Operations at a total pressure of 200 pounds per square inch, and

3. Operations wherein the partial pressure of the monoolefin charge is greater than one atmosphere.

less than In the catalytic dehydrogenation of monoolefins to conjugated diolefins in accord with thei teachings of this invention, the variables time and temperature are so closely interrelated that it is impossible to consider either of them alone. In general, temperatures of from 550 C. to 700 C. may be employed, preferably temperatures in the range 600 C. to 675 C. Contact time may vary from 0.1 to seconds or more, this variable being closely related to temperature. When operating at high temperatures the contact time is preferably short, varying from say 0.1 to 2 seconds or more while at lower temperatures longer contact times are required. It is obvious, that with all other conditions constant, the contact time necessary to obtain a given amount of conversion is also a function of-the amount of hydrogen added to the monoolefinic charge.

The catalysts employed form no part of the instant invention; any contact material that behaves in a satisfactory manner may be used. Several satisfactory catalysts have already been indicated andfor convenience methods for preparing some of these will be outlined but it is to be understood that these catalysts form no part of the present invention.

A chromium oxide on alumina catalyst, suitable for the purposes of the instant invention, may be prepared as follows: Granules of alumina, for example, 8 to 14 mesh activated alumina granules. are immersed in a-solutionof ammonium dichromate or chromium trloxide. The strength of the solution is so adjusted that the alumina granules, after saturation, contain an amount of solution equivalent to approximately 10% chromium based on the dry alumina. After saturation, the granules are separatedfrom excess solution, are drained and then slowly dried, for example, by spreading in thin layers-in air. The dried material is calcined for one hour at a temperature of 640 0.; the resulting product represents the desired catalyst. A molybdenum oxide on alumina catalyst may be made similarly, substituting ammonium molybdate solution for the ammonium dichromate solution (or chromium trioxide solution). In this case, about 5% molybdenum based on the dry alumina is to be adsorbed by the alumina.

To further aid in the understanding of the in.-

' stant invention the following examples are given but it is to be understood that these are illustrative only and in no way limit the scope of the invention.

. Emample 1 One volume of mixed normal butenes and three volumes of hydrogen are mixed and compressed to a pressure of 100 pounds per square inch and passed through a heated reactor containing a chromium oxide on alumina catalyst at 675 C. The contact time was 3.2 seconds. At the end of five hours the catalyst was analyzed for carbon and it was found that 111 g. butadiene 1,3 had formed per gram of carbon deposited. In a parallel experiment at 300 pounds pressure, (other conditions constant), only 41. g. of butadiene 1,3

formed per'gram of carbon deposited.

Example 2 that 56 g'. pentadiene 1,3had formed per gram of carbon deposited. In a parallel experiment at 750 pounds pressure (other conditions constant) only 14 g.'pentadienel,3 formed per gram of carbon deposited.

Example 3 One volume mixed methyl butenes and three volumes hydrogen are mixed and compressed to a pressure of 65 pounds per square inch and passed through a heated reactor containing a chromium oxide on alumina catalyst at 625 C. The contact time was 2 seconds. After 5 hours on stream the catalyst was analyzed for carbon and it was found that 21 g. isoprene had formed per gram of carbon deposited. In a parallel experiment at 200 pounds per square inch (other conditions constant) only 8 g. iso'prene per gram of carbon deposited was obtained.

An attempt was made to duplicate the experiments of Examples 1, 2 and '3 only at a total pressure of one atmosphere (actually slightly above one atmosphere due to pressure drop through the reactor).

Under such conditions carbon was depositedon the catalyst so rapidly that five hour runs could not be obtained, the catalyst becoming absolutely inactive long prior to the end of this period. I

It should be noted that the position of the double bond in the monoolefin chargedapparently has-no influence on the formifla'of the diolefin formed. Thus, both butene l and butene 2 yield butadiene 1,3. Similarly, both pentene 1 and pentene 2 yield pentadiene 1,3. Isoprene is obtained on dehydrogenating any of the isomeric methyl butenes. Evidently, during the dehydrogenation migration of a methyl group'is necessary to give the configuration necessary for forming conju gated diolefin. Presumably an isomerization of this type is more difilcult than one involving the mere shifting of a double bond.

The' described process is more than self sustaining with respect to hydrogen. By subjecting the reaction products to known separation processes, for example, fractionation at low temperatures or the well known adsorption-fractionation The partial pressure of the-olefin was accordingly about 0.25 atmosphere chromium oxide on alumina, molybdenum oxide' on alumina or similar contact agents. The reaction products, after addition of more hydrogen if desired, are passed to a second dehydrogenation zone for the conversion of monooleilns to dioleflns. The products from this second zone are preferably subjected to known separation means so as to eliminate hydrogen, unconverted paraiiins plus unconverted monooleflns and conjugated dioletlns as separate products. The requisite amount of hydrogen may be recycled to the second dehydrogenation zone while the paraflln-monoolefin mixture may be recycled to the first dehydrogenation zone. Or, if desired,

the final reaction products may be separated into hydrogen, unconverted paramns, uncon' verted monooleilns and conjugated dioleilns. The requisite amount of hydrogen and all of the unconverted monooleflns are recycled to the second dehydrogenation zone while unconverted is very simple, it is not particularly flexible.

paramns are recycled to the first dehydrogenation zone.

Another variation consists in separating the reaction products from the first dehydrogenation zone into unconverted paraflins, hydrogen and monooleflns. The unconverted parafllns are recycled to the first dehydrogenation zone while the hydrogen and monoolefins are mixed, additional hydrogen being added if'desired, the whole is sent to the second dehydrogenation zone. The

, reaction products from this second zone may be separated by known means into hydrogen, unconverted oleilns and conjugated dioleflns. The requisite amount of hydrogen and all of the unconve monooleflns may then be recycled to the second dehydrogenation zone.

If desired, the dehydrogenation of the parafilns to produce monooleflns may occur in the presence of hydrogen in which case it is preferable to operate under superatmospheric pressure. When the first dehydrogenation zone is operated in this manner certain changes in the separation and recycling of products from both the first and second dehydrogenation zones are usually desirable, but the nature of these changes will be obvious to those skilled in the art and need not be described further.

If desired, the production of conjugated diolenns from parailins may be accomplished by using but one catalytic dehydrogenation zone. By this procedure, the iormation'of monooleflns from paraflilns and the formation of conjugated dioleilns from monooleflns occur in the same catalyst bed. Hydrogen may be added with the par-amnic charge or may be introduced into the catalyst bed at or near the midpoint thereof. Or, if desired, the hydrogen obtained in the formation of the monocleilns may alone be used in the second dehydrogenation reaction. reaction products may be separated into hydrogen, unreacted paraflins, unreacted monopredetermined part of the hydrogen may be recycled either to the entrance of the catalyst bed or to or near the midpoint thereof. The unreacted monooletlns may be handled in a similar fashion while the unreacted paramns are recycled to the entrance of the single catalyst zone. While this single catalyst bed apparatus As the paramn to monoolefln reaction and the monolenn to conjugated diolefln reaction proceed at diflerent rates and are best conducted under different operating conditions, it is evident that a one zone system is not as well suited for the process as the more complicated but more flexible two zonesystem.

Even when operating in accord with the teachings of this invention the catalysts employed sooner or later decline in activity untilthe conversion of monooleflns to diolenns falls below economic limits. when this occurs, the catalysts may be subjected to a regeneration process. This regeneration may be accomplished, for-example, by treating the exhausted catalysts with air or dilute air at elevated temperatures. If desired, two catalyst chambers may he em- .ployed, one being used in converting monoolefins to conjugated dioleilns while the other is being regenerated, the function of the two chambers being reversed periodically as required.

While the present invention has been described in connection with details and specific examples thereof it is not intended thot these shall be regarded as limitations upon the scope of the invention except insofar as included in the following claims.

I claim:

1. In the manufacture of conjugated dioleflns from monooleflns containing more than three carbon atoms to the molecule, the steps com-v prising adding from one to ten volumes of hydrogen to said monoolefins, compressing the resulting mixture to a pressure such that the partial pressure of the monoolefin is greater than one atmosphere and the total pressure of the mixture is less than 200 pounds per square inch and subjecting the resulting compressed mixture 3 to catalytic dehydrogenation at elevated temperatures.

2. In the manufacture of conjugated dioleilns from monooleflns containing four to five carbon atoms to the molecule, the steps comprising adding from one to ten volumes of hydrogen to said monooleflns, compressing the resulting mixture to a pressure such that the partial pressure J of the monoolefln is greater than one atmosphere and the total pressure of the mixture is less than 200 pounds per square inch and sub-' jecting the resulting compressed mixture to catalytic dehydrogenation at elevated temperatures.

3. In the manufacture of conjugated diolenns from monooleilns containing four to five carbon atoms to the molecule and having at least four carbon atoms arranged in a straight chain, the steps comprising adding from one to ten volumes of hydrogento said monooleflns, compressing the resulting mixture to a pressure such that the partial pressure of the monoolefln is greater than one atmosphere and the total pressure of the mixture is less than 200 pounds per square inch and subjecting the resulting compressed mixture to catalytic dehydrogenation at elevated temperatm'es.

4. In the manufacture of butadiene 1,3 from cleans and conjugated diolenns. If desired, a u a normal'bute'ne, the steps comprising adding from one to ten volumes of hydrogen to said normal butane, compressing the resulting mixture to a pressure such that the partial pressure of the normal butene is greater than one is less than 200 pounds per square inch and subjecting the resulting compressed mixture to catalytic dehydrogenation at elevated temperatures.

6.-In the manufacture of isoprene from methyl butene, the steps comprising adding one to ten volumes of hydrogen to said methyl butene, compressing the resulting mixture to a pressure such that the partial pressure 01 the methyl butene is greater than one atmosphere and the total pressure of the mixture is'less than 200 pounds per square inch and subject- ,ing the resulting compressed mixture to catalytic dehydrogenation at elevated temperatures.

ROBERT F. RUTHRLUFF. 

